Dispersion compensation fibre

ABSTRACT

The phase modulator in the laser resonator requires both at the input and at the output defined linearly polarized light which has to be generated. Hitherto, this was bought about in a complex and expensive manner using polarization actuators. The invention provides a combination of dispersion compensation and definition of the linear polarization state in a dispersion compensation fiber (PM-DC fiber) which is birefringent at such a high level that only the two orthogonal linear polarization states can be diffused therein. The invention can be used for mode-locked fiber lasers in which the time pulse width has to be as small as possible.

BACKGROUND INFORMATION

[0001] The present invention relates to dispersion compensation fibers of the type characterized in greater detail in the preamble of Patent claim 1. They were described, for example, by D. M. Pataca, M. L. Rocha, K. Smith, T. J. Whitley, R. Wyatt in “Actively modelocked Pr³⁺-doped fluoride fibre laser” Electr. Lett., 30 (1994) 2, p. 964p.

[0002] The use and design of such dispersion compensation fibers (DC fibers) for compensation of the dispersion of the active fibers of the fiber laser with a fiber laser structure such as that presented in FIG. 1, for example, is already known. The phase modulator in the laser resonator requires both at the input and at the output defined linear polarized light which must be produced. Previously, this has been achieved using polarization convertors, which is generally complex and laborious.

[0003] As the radiation source of ultra-high bit rate transmission systems and as a source of solitons, modelocked fiber lasers are used to advantage. The most important prerequisite is that the pulse width over time must be as small as possible, i.e., it must not exceed a few ps. Various authors, such as D. M. Pataca, M. L. Rocha, K. Smith, T. J. Whitley, R. Wyatt: “Actively modelocked Pr³⁺-doped fluoride fibre laser” Electr. Lett., 30 (1994) 2, p. 964p, have demonstrated that the chromatic dispersion of the active fibers of the fiber laser has a pulse widening effect. The formulas by D. J. Kuizenga, A. E. Siegman: “FM and AM Mode Locking of the Homogeneous Laser—Part I: Theory.” IEEE J. Quant. Electr. 6 (1970), p. 694 with the supplement by G. Geister: “Integrierte optische Modulation von Nd-Faserlasem” [Integrated optical modulation of Nd fiber lasers] Fortschrittsberichte VDI Reihe [VDI Progress Reports Series] 10 (1990) 140, 1, 102 describe the dependence of pulse half-width τ_(p) over time on modulation frequency f_(m), modulation index δ_(c), laser wavelength λ, length of active fiber L_(a), gain coefficient g and spectral half-width Δλ of the fluorescence spectrum: $\tau_{p} = {{\frac{\sqrt{2\sqrt{2}\ln \quad 2}}{\pi}\left\lbrack \frac{1}{f_{m}^{2}\delta_{c}} \right\rbrack}^{1/4}\left\lbrack {\left( {\frac{\lambda^{2}L_{a}}{2\pi \quad c}D} \right)^{2} + \left( \frac{g}{\pi^{2}\Delta \quad f^{2}} \right)^{2}} \right\rbrack}^{1/8}$

[0004] The extension by Geister is expressed by the additional term with D, taking into account chromatic dispersion.

[0005]FIG. 2 shows the negative influence of dispersion D on pulse half-width for the case of a Pr³⁺ ZBLAN glass fiber laser. This means that D must disappear in order to minimize τ_(p), i.e., dispersion must be compensated. This can be accomplished by using a chirped fiber Bragg grating as the laser reflector, e.g., as the decoupling reflector. However, this method is very problematical. The reflecting power of the chirped fiber grating is precisely defined by optimizing the fiber laser and must thus be verified because otherwise the laser threshold is increased and the output power is reduced. To compensate for the dispersion of the active fibers the spectral half-width must be >10 nm according to estimates; otherwise τ_(p) is increased. These technological requirements cannot be met at the present time.

[0006] Therefore, the object of the present invention is to permit a combination of the effects of dispersion compensation and definition of the linear polarization state in a glass fiber that has high birefringence of the DC fibers and is linked to the active fiber.

[0007] This object is achieved with the means indicated in the characterizing part of Patent claim 1.

[0008] Advantageous possible refinements of the present invention are mentioned in the characterizing parts of Subclaims 2 and 3.

[0009] The present invention with its effects and advantages is explained in greater detail below on the basis of exemplary embodiments. The respective drawings and tables show:

[0010]FIG. 1: the structure of a Pr³⁺ doped fluoride glass fiber laser with two PM-DC fibers;

[0011]FIG. 2: the measured and calculated pulse half-width with and without taking into account dispersion as a function of modulation frequency;

[0012]FIG. 3: an n(r) profile of the stepped index fiber 79 M;

[0013]FIG. 4: an n(r) profile of the W-profile fiber 82 M;

[0014]FIG. 5: a refractive index profile of the two TC profile fibers with a rectangular core (top) and with a parabolic core (bottom); preform measurement;

[0015]FIG. 6: a core geometry of the W-profile fiber (elliptical fit); large axis: 2.48 μm, small axis: 1.54 μm;

[0016]FIG. 7: a group transit time curve and dispersion curve for the stepped index fiber 79 M;

[0017]FIG. 8: the chromatic dispersion of the W-profile fiber 82 M in a Raman fiber laser measurement;

[0018]FIG. 9: the dispersion curves of the two TC-profile fibers 94 M and 96 M with a rectangular core (top) and with a parabolic core (bottom);

[0019]FIG. 10 a double-pulse diagram on an oscilloscope screen; pulses of a stepped index fiber, fiber length 615 m, λ=1550 nm, pulse interval 1679→PMD=2.63 ps/m;

[0020] Table 1: measurement results for the chromatic dispersion of the two PM-DC fibers 79 M and 82 M, measurements with the Raman fiber laser and the Mach-Zehnder interferometer;

[0021] Table 2: PMD, group birefringence and group beat length of two PM-DC fibers and three commercial PM fibers for comparison; and

[0022] Table 3:

[0023] geometric data, attenuation, dispersion measurement results and PMD values of the two TC-profile fibers with a rectangular and parabolic core profile.

[0024] The present invention concerns DC fibers with a high birefringence, compensating for the dispersion of the active fibers as well as defining, fixing and controlling the polarization state of the radiation. These fibers are referred to below as PM-DC fibers.

[0025] This yields an all-fiber method of achieving the object according to the present invention with low resonator losses, because these PM-DC fibers have a low attenuation. Thus, a low laser threshold and high output power can be expected. In addition, a very compact design is guaranteed.

[0026] Dispersion compensation fibers with a high birefringence (PM-DC fibers) can be a very useful tool in optical measurement and sensor technology in cases where it is important to compensate for the chromatic dispersion of (active) glass fibers while at the same time defining, fixing and controlling the polarization status of the radiation. This is to be illustrated on the basis of an example.

[0027] The discussion below is limited to the 3^(rd) optical window (at λ=1550 nm). The high birefringence is achieved due to a marked ellipticity of the fiber core, which is called geometric birefringence.

[0028] Four fiber profiles which meet this requirement are described below. An iterative, inverse method was presented by R. Boness, W. Nowak, J. Vobian, S. Unger, J. Kirchhof in “Tailoring of dispersion-compensation fibers with high compensation ratios up to 30,“Pure Appl. Opt. 5 (1995) 333, which makes it possible to determine the relevant fiber parameters and optimize the fiber profile for given dispersion values and Petermann II field radii. This method uses the Levenberg-Marquardt algorithm based on the Helmholtz equation and the singular Sturm-Liouville eigenvalue solution.

[0029] In the present case, the dispersion in the 3^(rd) optical window must assume high negative values.

[0030]FIGS. 3, 4 and 5 show the fiber profiles of a stepped index fiber, a W-profile fiber and two triple-clad (TC) fibers with rectangular and parabolic core profiles. FIG. 6 shows the geometry of the core of the W-profile fiber.

[0031] The measurements of chromatic dispersion D(λ) can be performed with a Raman fiber laser RFL, described in J. Vobian: Dispersionsmessungen zur Charakterisierung von Quarzglasfasem der optischen Nachrichtentechnik. Mitteilungen aus dem Forschungsinstitut der Deutschen Bundespost [Dispersion measurements to characterize quartz glass fibers of optical communications technology. Reports from the Research Institute of the Federal German Post Office], No. 14 (1992), and the Mach-Zehnder interferometer MZI, described in J. Vobian: Chromatic and Polarization Dispersion Measurements of Single-Mode Fibers with a Mach-Zehnder Interferometer between 1200 and 1700 nm, J. Opt. Commun. 11 (1990), 1, p. 29.

[0032] The advantages of this measurement technique include the large measurement range of up to λ≧1700 nm, the large number of measurement points, the high measurement accuracy, the possibility of running through the λ spectrum continuously and without any gap, and in the case of the MZI, performing measurements on short pieces of fiber. It is readily possible to measure very high negative dispersion values in the 3^(rd) optical window in the case of RFL, in the case of MZI, this is also feasible with the help of modifications in the measurement setup.

[0033]FIG. 7 shows the group transit time curve and the dispersion curve for the stepped index fiber 79 M; FIG. 8 shows the dispersion curve of the W fiber 82 M, and FIG. 9 shows he dispersion curves of the two TC fibers 94 M and 96 M. Table 1 gives the relevant data on the two fibers 79 M and 82 M. The deviations between RFL and MZI measurements can be explained by fiber inhomogeneities.

[0034] Table 2 summarizes the geometry and dispersion data on the two TC fibers 94M and 96M. The birefringence values will be discussed later.

[0035] The measurements of birefringence and polarization mode dispersion (PMD) are performed in the time domains with a Michelson interferometer, as described by N. Gisin, J. -P. Von der Weid, J. -P. Pellaux in: Polarization Mode Dispersion of Short and Long Single-Mode Fibers. J. Lightw. Technol. 9 (1991) 7, p. 821, and by J. Vobian, K. Mörl, W. Keilig in: Different aspects of the polarization mode dispersion measuring technique. Proc. EFOG&N 94, Heidelberg 1994, p. 174.

[0036]FIG. 10 shows how the pulses are splintered due to the high birefringence causing high PMD values. The interval between the two pulses is a measure of the birefringence of the fibers. Table 3 summarizes the measurement results for fibers 79 M and 82 M. It also gives the corresponding data on commercial fibers that maintain polarization (HiBi) for comparison purposes (PANDA, YORK bow-tie and AT&T fibers). It can be seen here that the birefringence of the two fibers 79 M and 82 M with elliptical cores is much higher, and the beat lengths are correspondingly smaller. The relationships between group birefringence B_(G), group length l_(G) and polarization mode dispersion PMD (Δτ_(GP)) are shown below: ${{\Delta \quad \tau_{GP}} = {\frac{\Delta \quad t_{GP}}{L} = \frac{B_{G}}{c}}},{I_{G} = {\frac{\lambda}{B_{G}}.}}$

[0037] Table 2 also shows, as already mentioned, the measured birefringence and PMD values for the two TC fibers. Both the dispersion values and the birefringence and PMD values are smaller for the two TC fibers than for the stepped index and W fibers. That was intentional, because this information is completely sufficient, but in addition, the attenuation values of the two TC fibers are much lower, which has a positive effect on laser operation, as mentioned above. TABLE 1 Measurement results for chromatic dispersion of the two PM-DC fibers 79M and 82M D D D (1300 nm) (1540 nm) (1550 nm) Elliptical Measure- in in in fit PM-DC ment ps/km · ps/km · ps/km · large/ fiber setup nm nm nm small axis W fiber Raman −150.7 −194.7 −197.8 2.48/1.54 μm 82M fiber laser W fiber Mach −148.3 −192.2 −195.3 82M Zehnder interfero- meter Stepped Raman −160.5 −105.0 −102.2 2.24/1.26 μm index fiber fiber laser 79M Stepped Mach- −154.8 −101.2 −98.8 index Zehnder fiber interfero- 79M meter

[0038] TABLE 2 PMD, group birefringence and group beat length of two PM-DC fibers and of three commercial PM fibers for comparison Wavelength in nm Parameter Fiber 1276 1321 1516 1546 PMD in W fiber 82M 5.64 5.91 8.68 8.89 ps/m Stepped index fiber 79M 3.34 3.28 2.67 2.56 (2.63) PANDA fiber 1.44 1.47 — YORK bow-tie fiber 2.18 2.22 — AT&T PM fiber 0.77 0.78 — Group W fiber 82M 16.92 17.73 26.04 26.67 bire- Stepped index fiber 79M 10.02 9.84 8.01 7.68 fringence PANDA fiber 4.32 4.40 — B_(G) · 10⁻⁴ YORK bow-tie fiber 6.54 6.66 — AT&T PM fiber 2.30 2.35 — Group W fiber 82M 0.75 0.75 0.58 0.58 beat Stepped index fiber 79M 1.27 1.34 1.89 2.01 length PANDA fiber 3.01 3.42 — in nm YORK bow-tie fiber 1.97 2.25 — AT&T PM fiber 5.58 6.46 —

[0039] TABLE 3 Geometric data, attenuation, dispersion measurement results and PMD values for the two TC profile fibers with a rectangular core and a parabolic core DC fiber 94M DC fiber 96M a in μm 5.2 5.56 r₁ in μm 1.46 1.52 r₂ in μm 4.43 4.75 α (1300 nm) dB/km 2.0 4.6 α (1550 nm) dB/km 1.3 2.7 D (1300 nm) ps/km · nm −33.9 −43.0 D (1550 nm) ps/km · nm −40.0 −41.9 S (1550 nm) ps/km · nm² −0.0264 −0.0129 PMD (3121 nm) in ps/m 0.24 0.62 B_(G) (1321) 10⁻⁴ 0.72 1.86 I_(BG) (1321) in cm 1.83 0.71 PMD (1546) in ps/m 0.36 0.89 B_(G) (1546) 10⁻⁴ 1.08 2.65 I_(BG) (1546) in cm 1.43 0.58 

1. Dispersion compensation fiber for compensation of dispersion in a glass fiber, which is accommodated in the fiber laser resonator and linked to the active fiber, characterized in that it is formed by a controlled elliptical shaping of the core for geometric birefringence which due to its refractive index profile allows only the propagation of the two orthogonal as well as linear polarization states, both for dispersion compensation for definition of the linear polarization state, and in that this refractive index profile is determined not only by the elliptical shaping but also by a selected doping in the preform.
 2. Dispersion compensation fiber according to claim 1, characterized in that the refractive index profiles are designed for high negative dispersion values in the 3^(rd) optical window as well as high positive dispersion values in the 2^(nd) optical window with regard to waveguide and material dispersion, in order to be able to compensate dispersion of the active fibers in both optical windows and fix the polarization state.
 3. Dispersion compensation fiber according to claim 1, characterized in that in accordance with the final consequence of minimizing resonator losses in fiber lasers, the dimensioning of the minimum length of the dispersion compensation fiber is adapted experimentally and mathematically for both effects, i.e., primarily to the dispersion compensation and secondarily to the definition of the linear polarization state. 